Hybrid electric vehicles (HEVs) include both an engine and at least one electric motor to provide drive torque to the wheels. There are various types of HEVs. For example, “parallel” hybrid vehicles typically include clutches that selectively enable either or both the engine and the motor to provide drive torque. A “series” hybrid vehicle typically includes an electric motor that is always driveably connected to the road wheels, and an engine that is not mechanically connected to the wheels. In other words, the engine does not provide any of the torque required to propel the vehicle. Rather, in a “series” hybrid vehicle, the engine powers a generator to produce electrical energy that is stored in the battery and/or used by the motor.
Any type of HEV can be equipped with a cruise control system in which an operator can define a set speed for the vehicle to travel. When cruise control is activated by an operator, the HEV's control system maintains the vehicle speed at the set speed by controlling the engine and the electric motor to provide an adequate amount of combined torque to the wheels. If the motor alone can provide adequate torque to maintain the speed of the HEV, the engine can be turned off to conserve fuel. The engine may later have to be started if, for example, additional torque is necessary that cannot be fulfilled by the electric motor alone. Excessive engine stopping and starting during cruise control can impact drivability and NVH of the vehicle.